1. Technical Field
The present invention relates to a fuel injection device for a diesel internal combustion engine. A pressure line connects an injection nozzle to a fuel injector The fuel injector has an injector body with at least one fuel line for the inflow and outflow of fuel. The injector has a pump chamber and a plunger. The plunger is connected with a drive assembly. The drive assembly includes an axially movable piston which is acted on by fluid pressure.
2. Description of the Prior Art
A great number of fuel injection devices for diesel internal combustion engines are known. In most of these devices, the plunger is driven by a camshaft. From Swiss Pat. No. 539,778 is known a fuel injection device in which the driving of the plunger takes place by means of a drive assembly having an axially movable piston which is acted on by fluid pressure. This device includes a delivery pump for the fuel, which is part of a device for the feeding of fuel to one or more injection nozzles. The delivery pump is connected with a reservoir and a pressure-regulating valve, which regulates the delivery pressure in the fuel conduction system. From the delivery pump, the fuel is conducted to an electro-magnetically actuated hydraulic valve, to a slide valve, to a servopiston and to the injection nozzle. The electro-magnetically actuatable hydraulic valve is connected with an electric control device, which communicates control signals for the beginning and end of the injection cycle. The slide valve is connected with the electro-magnetically actuatable hydraulic valve, and has two control edges which control the inflow of fuel to a working surface of the servopiston. The land of the slide valve is acted on by fuel at the pressure of the delivery pump and by a spring force which is less than the force exerted by the delivery pressure of the pump. The servopiston is connected directly with the plunger of the fuel injector, while the plunger conveys fuel from a pump chamber to the injection nozzle. The volume of fuel flowing into the pump chamber is determined through the pressure prevailing on the delivery pump and a restrictor located in the line.
At the beginning of the injection cycle, a control signal brings the electro-magnetically actuatable valve into a position in which the slide valve is subjected to pressure by the delivery pump. The land of the slide valve unblocks a passage from the fuel line to the working surface of the servopiston. The servopiston and, thus, the plunger are moved, and the injection process is started. At the moment desired for the end of the injection cycle, the electric control device communicates a second signal to the electro-magnetically actuatable valve. The electromagnetically actuatable valve moves to a different switch position, and the slide valve is relieved of fuel pressure from the delivery pump. The land of the slide valve is pushed by the spring force, and unblocks a passage connecting the working surface of the servopiston with a pressureless return line of the fuel system. The injection stroke is interrupted and the plunger, and the servopiston, are driven back by the delivery pressure in the fuel system. The return distance of the servopiston is determined by the volume of fuel flowing, which in turn, is predetermined by the restrictor located in the inflow. At partial capacity, the servopiston remains floating at the end of the filling stroke. That is, the servopiston is not stationary. In this inexact position of the servopiston, the fuel injection device is ready for a new injection stroke.
Fuel injection devices of the kind described make possible, through the installation of the slide valve, the use of relatively small electro-magnetically actuatable valves. But they have the disadvantage that the exact amount of the injection volume is beset with difficulties. The electric control and the whole fuel system must be very exactly adapted to each other, especially in high speed diesel engines, in order to introduce the fuel, in the right amount and at the right timing, into the combustion chamber of the diesel internal combustion engine. This tuning is difficult and involves great technical expense. Since the servopiston, within a great load range, does not start from the same stationary position during each injection cycle, the fuel amount is nevertheless very inexact. The floating servopiston cannot be exactly positioned because of leakage losses and changes in fuel viscosity cause a different degree of filling. The electric time control cannot detect and compensate for these variations. As a result, within several injection cycles, different fillings occur. The known injection device has, moreover, no backup, and the injection process cannot be carried out with failure of the electric control. The hydraulic system has a different characteristic according to the kind of fuel used, and sometimes the functionality of the hydraulic element is no longer assured